Video data in a digital format offers many advantages over the conventional analog format and has become the dominant format for video storage and transmission. The video data is usually digitized into an integer represented by a fixed number of bits, such as 8 bits or 10 bits per sample. Furthermore, color video data is often represented using a selected color system such as a Red-Green-Blue (RGB) primary color coordinates or a luminance-chrominance system. One of the popularly used luminance-chrominance color system used in digital video is the well-known YCrCb color system, where Y is referred to as the luminance component and Cr and Cb are referred to as the chrominance signals. The RGB components and the Y component are associated with the intensity of the primary colors and the luminance respectively, and their digital representation is often mapped to an integer having a range associated with the number of bits of the digital representation. For example, an 8-bit video data usually represents an intensity level 0 to 255. On the other hand, the chrominance components, Cr and Cb, correspond to difference data and their digital representation is often mapped to an integer having negative and positive values. For example, an 8-bit Cr/Cb data represents a chrominance level from −128 to +127.
Along the processing path in a digital video system, the processed video data may become exceeding the original range of digital representation or the mean value of the processed video may be shifted. For example, various digital filters may be applied to digital video data for different purposes such as noise suppression, edge sharpening, block boundary smoothing, anti-aliasing, and etc. Particular coefficients selected for the filter will affect the intensity range of the processed video data and cause unexpected artifact if the out of range data is not taken care of properly. Data overflow and underflow is a symptom often occurred when the out of range data is not taken care of properly, where an extremely bright pixel may become a dark pixel and vice versa. In some cases, the mean intensity level of processed video data may also be shifted. The intensity shifting may be caused by filtering, data rounding, quantization or other processing. Intensity shift, also called intensity offset, may cause visual impairment or artifacts, which is especially noticeable when the intensity shift varies from frame to frame. Therefore, the pixel intensity has to be carefully clipped or offset corrected to avoid the potential problems mentioned above.
However, picture contents often are very dynamic between frames and within a frame and change from time to time. In order to explore the advantage of content adaptive pixel processing in a dynamic video environment, it is desirable to develop a system and method that can select a picture unit, termed predefined set, for the underlying video data to further improve the quality of processed video data. The predefined set can be selected to capture the dynamic characteristics of the underlying video data.